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    Vernier sextant adjustment?
    From: Doug Royer
    Date: 2003 May 8, 10:13 -0700

    I was looking through an old navigation book last night and came to a
    discriptive paragraph of telescope adjustment for a sextant.This book was
    written in 1918 and all the illustrations of the sextants were of the
    vernier type.I was hopeing some of you who are familiar with older
    instruments can understand what the instructions mean and can explain them.I
    will write it directly from the book.
    From "Elements of Navigation and Pilotting" by Lt. W.J. Henderson, A.M. :
    section IV. The line of sight of the telescope must be parallel to the plane
    of the instrument."Screw in the telescope containing the two parallel
    wires,and see that they are turned untill parallel with the plane of the
    sextant;then select 2 stars,at least 90* apart,and make an exact contact at
    the wire nearest the plane of the instrument,and read the measured
    angle.Move the sextant so as to throw the objects on the other wire,and if
    the contact is still perfect,the axis of the telescope is in its right
    situation and the telescope adjustment is correct.If the images have
    seperated,it shows that the object end of the telescope droops toward the
    plane of the sextant,and if the images overlap, it proves that the object
    end of the telescope points away from the plane of the instrument.This will
    be rectified by the screws in the collar of the sextant.A defect in the
    telescope adjustment always makes angles too great"(Patterson)
    What are the wires he talks about?What is accomplished by reading the
    measured angle?

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